1. Field of the Invention (Technical Field)
The present invention relates to modifying zeolite, montmorillonite minerals, activated carbon and other materials in order to increase their affinity for arsenic species, and their use in selectively removing arsenic species from an aqueous medium. The preferred modifiers are water soluble zirconium-containing chemicals, e.g., zirconyl chloride, zirconium acetate, and zirconyl nitrate.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Arsenic has long been known as a highly toxic element. In order to reduce the public health risks from arsenic in drinking water, the United States Environmental Protection Agency (EPA) recently set the arsenic standard for drinking water at 10 ppb down from an original 50 ppb, emphasizing the need for more efficient and cost-effective means for water treatment to remove trace arsenic.
Arsenic exists in two soluble and dangerous oxidation states, As3+, which is known as arsenite, and As5+, which is known as arsenate. Both forms are toxic and exist in groundwater, although arsenite is the more lethal and the more difficult to remove.
Higher levels of arsenic are generally to be found in ground water rather than in surface water sources of drinking water. The western states of the United States, e.g., New Mexico, have a great number of water systems with arsenic levels greater than 10 ppb. Therefore, to meet the new As standard for water, it is imperative to seek other cost-effective, easy-to-use technologies that could effectively remove As from water.
Conventionally various techniques have been examined to remove As from water, such as precipitation (e.g., salts of iron, aluminum, or copper) and coagulation and filtration processes. However, these conventional methods are generally unable to successfully remove the As to lower levels due to the affinity and solubility limitation of the resultant products. The procedures are also time-consuming and expensive, and so not cost-effective.
One method for removing arsenic species from an aqueous medium is the use of an alumina sorbent, as discussed in U.S. Pat. No. 5,556,545, to Volchek, et al. However, the method has some inherent limitations, requiring regeneration and conditioning of the sorbent. Therefore, this regeneration process creates a hazardous solution. Furthermore, the regeneration process results in loss of the sorbent, thus increasing the cost of using activated alumina as a method for removing arsenic from an aqueous medium.
Another method to remove arsenic species from an aqueous medium is ion exchange. One of the disadvantages of this process is that the ion-exchangers utilized are mostly synthetic resins and hence are very expensive. See, e.g., Japanese Patent Application Publication No. H06-304573, to Masafumi, et al. Furthermore, few resins are selective in arsenic removal. A variety of anions such as sulfates compete for the ion-exchange sites in the resin. In general, ion-exchange is not a feasible method of removing arsenic from an aqueous medium if the medium contains a high level of dissolved solids or sulfate concentrations.
Another method for removing arsenic species from an aqueous medium is through the use of a membrane process. A membrane process involves passing the aqueous medium through the membrane to filter the selected material. However, membrane processes are costly as a method for removing arsenic species from an aqueous medium.
A recently disclosed method is the use of zirconium-impregnated resin as disclosed in U.S. Pat. No. 6,077,809, to Suzuki, et al. The adsorbent material consists of a porous carrier material such as crosslinked polyacrylate resin beads and a crystalline hydrous zirconium oxide impregnating the pores of the carrier in the monoclinic or cubic crystal form. The adsorbent material is prepared by soaking the carrier material with an alcohol solution of a zirconium compound such as zirconium oxychloride to impregnate the pores with the zirconium compound, followed by hydrolysis of the zirconium compound with an aqueous alkaline solution to convert the same into zirconium hydroxide and subjecting the carrier material impregnated with zirconium hydroxide to a hydrothermal reaction under specific conditions to convert the zirconium hydroxide into crystalline hydrous zirconium oxide which has a monoclinic or cubic crystal form depending on the acidity or alkalinity of the aqueous medium employed in the hydrothermal treatment. This method is not only complicated and very expensive but also involves harsh synthesis conditions. Additionally, the regeneration process creates hazardous solutions.
Another recently disclosed method is the use of pure zirconium hydroxide as a paste in water filters, as disclosed in U.S. Pat. No. 6,383,395, to Clarke, et al. The media includes a material selected from zirconium hydroxide, titanium hydroxide, hafnium hydroxide and combinations thereof. The media is preferably in powder form when used to treat water. The media needs to be regenerated repeatedly in order to reduce the cost, while it creates hazardous solutions that need to be disposed of at a cost. Because the media used is in the form of a paste, it does not have high hydraulic permeability and requires use at high pressure. This significantly limits the use of the material to small, high-pressure systems.
Zirconium-loaded activated charcoal has been used in analytical procedures as an adsorbent material for preconcentrating inorganic compounds of As(V), Se(IV), Se(VI), and Hg(II) in aqueous solutions.
The present invention employs materials comprising zirconium bound to zeolite, montmorillonite, activated carbon, fly ash, and like other materials (e.g., cellulose acetate or a cation-exchangeable clay mineral other than montmorillonite, or mesostructured materials (e.g., silica)) for removing arsenic species from an aqueous medium. Zirconium is reported to be environmentally benign, having low biotoxicity and is relatively inexpensive. Both zeolite and bentonite minerals are naturally occurring, inexpensive minerals. They are ubiquitous in the western states of the United States of America.
Zeolites and smectite contain a net negative charge due to isomorphous substitution in the aluminosilicate layers. In nature, this charge is neutralized by inorganic cations such as Na+ or Ca2+ on the clay interlayers and external surfaces. Hydration of these cations in the presence of water initiates a separation of the smectic clay layers causing a swelling of the clay. Zeolites have a generally three-dimensional open framework with channels that accommodate water molecules and cations.
Both zeolite and bentonite (mainly montmorillonite) clay minerals are used widely in the construction of liners for hazardous waste landfills, slurry walls, industrial waste treatment lagoons, sewage lagoons, and tank forms. Activated carbon has been used widely in water treatment due to its high surface area, low cost, and sorptive properties for many compounds.
Other background materials include U.S. Pat. No. 4,046,687, to Schulze; P. Bermejo-Barrera, et al., xe2x80x9cA Comparison of different chemical modifiers for the direct determination of arsenic in sea water by electrothermal atomic absorption spectrometryxe2x80x9d. Fresenius Journal Of Analytical Chemistry 355(#2):174-179 (1996); Y. L. Chen, et al., xe2x80x9cDetermination of arsenic(v) and arsenic(iii) species in environmental-samples by coprecipitation with zirconium hydroxide and pre-atomization atomic-absorption spectrometryxe2x80x9d, Journal Of Analytical Atomic Spectrometry 8(#2):379-381 (1993); E. Farfantorres, et al., xe2x80x9cPillared clays: preparation and characterization of zirconium pillared montmorillonitexe2x80x9d, Catalysis Today 15(#34):515-526 (1992); S. Peraniemi, et al., xe2x80x9cOptimized arsenic, selenium and mercury determinations in aqueous-solutions by energy-dispersive X-ray-fluorescence after preconcentration onto zirconium-loaded activated-charcoalxe2x80x9d, Analytica Chimica Acta 302(#1):89-95 (1995); T. Viraraghavan, et al., xe2x80x9cArsenic in drinking water: Problems and solutionsxe2x80x9d, Water Science And Technology 40(#2):69-76 (1999); S. Yamanaka, et al., xe2x80x9cHigh Surface-area solids obtained by reaction of montmorillonite with zirconyl Chloridexe2x80x9d, Clays And Clay Minerals 27(#2):119-124 (1979); Y. W. You, et al., xe2x80x9cRemoval of arsenite from aqueous solutions by anionic claysxe2x80x9d, Environmental Technology 22(#12):1447-1457 (2001). The high surface-area material produced by reacting montmorillonite with zirconyl chloride is useful for catalysis reactions.
The present invention is of a method for removing contaminant species from an aqueous medium, comprising: providing a material to which zirconium has been added, the material selected from one or more of zeolites, cation-exchangeable clay minerals, fly ash, mesostructured materials, activated carbons, cellulose acetate, and like porous and/or fibrous materials, and contacting the aqueous medium with the material to which zirconium has been added. In the preferred embodiment, the method removes one or more species of arsenic, more preferably all arsenic species in the form of arsenate, arsenite and organometallic arsenic (with no pretreatment necessary, such as oxidative conversion of arsenite to arsenate). The method preferably selectively removes the arsenic species to below approximately 10 ppb, most preferably to below approximately 1 ppb. The material preferably has zirconium added via an aqueous solution of one or more compounds selected from zirconyl chloride (most preferred), zirconyl nitrate, and zirconium acetate (which solution may be preheated). Clinoptilolite and montmorillonite are the preferred materials to which zirconium is added. The aqueous medium is preferably groundwater, tap water, or wastewater and can have a wide-ranging pH of from approximately 2 to approximately 10. The contaminant is preferably one or more of the group consisting of arsenic, fluoride, selenium, phosphate, molybdenum, mercury, chromate, heavy metals, and radionuclides. The material to which zirconium has been added may be placed in any of the following: water filters, impermeable barriers, permeable barriers, liners, columns, filtration membranes, and synthetic geomembranes.
The invention is also of a composition of matter comprising a material to which zirconium has been added, the material selected from one or more of fly ash, mesostructured materials, cellulose acetate, and cation-exchangeable clay minerals other than montmorillonite.
The invention is additionally of a method of making a composition of matter useful for removing contaminant species from an aqueous medium, comprising: providing a material selected from one or more of zeolites, cation-exchangeable clay minerals, fly ash, mesostructured materials, activated carbons, cellulose acetate, and like porous and/or fibrous materials; and adding zirconium to the material. In the preferred embodiment, an aqueous solution of one or more compounds selected from zirconyl chloride, zirconyl nitrate, and zirconium acetate is employed to provide the zirconium, which solution may be preheated. The material is preferably placed in the solution for less than or equal to approximately three days. The material may then be placed in any of the following: water filters, impermeable barriers, permeable barriers, liners, columns, filtration membranes, and synthetic geomembranes. The material is preferably rinsed, sieved to remove fines, air dried, and/or calcined.
The invention is further of an apparatus for removing contaminant species from an aqueous medium, comprising: a material to which zirconium has been added, the material selected from one or more of the group consisting of zeolites, cation-exchangeable clay minerals, fly ash, mesostructured materials, activated carbons, and cellulose acetate; and means for contacting the aqueous medium with the material to which zirconium has been added. In the preferred embodiment, the material comprises clinoptilolite and/or montmorillonite. The contacting means preferably comprises one or more of the following: water filters, impermeable barriers, permeable barriers, liners, columns, filtration membranes, and synthetic geomembranes.
A primary object of the present invention is to develop cost-effective, easily manufactured and applied adsorbents capable of removing arsenic from drinking water, surface water groundwater and wastewater and that can be applied in a variety of treatment systems and configurations.
An advantage of the present invention is the ease of manufacturing the materials, very low cost, and ease of disposal. By simply immersing substrates such as zeolite, montmorillonite, or activated carbon in a zirconium-containing solution using the methods described herein, a new class of cost-effective adsorbents for arsenic (III), arsenic (V), organic arsenic and other contaminants is presented.
Another advantage of the present invention is that the materials can be easily engineered in granular form of different sizes. This permits design of column type containers for water treatment allowing the material to be used in small and very large treatment systems. The materials can also be used in the form of powders.
Yet another advantage of the invention is that no pretreatment of the water is necessary. The adsorbents have high selectivity for all three forms of arsenic: arsenate, arsenite, and organometallic arsenic.
Another advantage of the present invention is that it is very inexpensive in comparison with traditional methods for arsenic removal from water. The principles of the invention are suited to allow nearly complete removal of arsenic from drinking water, surface water, groundwater, and wastewater at minimum cost.
A further advantage of the present invention is that the zirconium-containing adsorbents (after removing, for example, arsenic) can be easily disposed in landfills without recycling.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.